This invention pertains to thermodynamically efficient internal combustion engines, particularly U.S. Patent Classifications 123/46R (free piston engines), 123/46A (free piston engines with two chambers and one piston), 123/46B (free piston engines-phasing means between two or more units) and 60/595 (power plants in which a free piston device supplies motive fluid to a motor). In addition to the field of free-piston engines, the invention is relevant to internal combustion engines employing homogeneous charge compression ignition (HCCI). It is also relevant to the efficient operation of internal combustion gas turbine engines. In one embodiment employing compressed air starting, the invention pertains to U.S. Patent Classification 60/596 (power plants using a free piston device with a pressure fluid starting means).
Two-stroke free-piston engines used in combination with a rotary, gas-driven motor (often referred to as “compound free piston-gas turbine engines,” of which the present invention is a subtype) are known and have been proposed as a means of improving fuel efficiency, reducing cost, and adding multifuel capability to conventional gas turbine engines. Such compound engines have also been proposed as a means of improving fuel efficiency, increasing power density, improving low-speed torque, reducing cost and complexity, and adding multifuel capability to conventional crank-piston internal combustion engines. Most early designs employed a free-piston combustor (gas generator) of the “two-piston opposed” type to generate pressurized gas, and used the pressurized gas to rotate an axial power turbine, as instanced by A. F. Underwood, “The GMR 4-4 ‘Hyprex’ Engine: A Concept of the Free-Piston Engine for Automotive Use,” SAE Transactions 65 (1957): 377-391. Known prototypes demonstrated an ability to run on a wide variety of liquid fuels, had very low vibration, and exhibited excellent low-speed torque. However, they proved to have fundamental structural weaknesses and were very bulky and heavy. Fuel efficiency was reported to be roughly equal to or slightly better than that of conventional crank piston engines.
A proposed improvement to the “two-piston opposed” model for compound free piston-gas turbine engines might be described as a “single-piston” or “single piston assembly” model, as instanced by U.S. Pat. No. 1,785,643, U.S. Pat. No. 2,963,008, and U.S. Pat. No. 4,205,528. These and similar designs incorporate a free-piston combustor (gas generator) in which a single, double-ended piston, or alternatively a single assembly consisting of two pistons fixedly attached to each other via a rigid connecting rod, oscillates back and forth in either a single cylinder or two coaxial cylinders, with combustion occurring at alternate cylinder ends (heads). The pressurized gas thus produced is used to rotate a power turbine to do useful work, while the piston motion itself is used solely to produce compression in the cylinder end opposite each combustion event. Such designs claim greatly increased operating frequency of the free-piston combustor (gas generator), as well as superior structural strength, greater simplicity, and dramatically reduced weight and size, relative to the “two-piston opposed” model.
Within the past twenty years, free piston engines of the single piston/single piston assembly type have been proposed as a means of obtaining homogeneous charge compression ignition (HCCI), which offers advantages over conventional combustion models in terms of near-instantaneous burn rate (hence nearly ideal constant volume combustion), the enabling of high compression ratios, the ability to run unthrottled at very lean mixtures, multifuel capability, and reduced particulate and NOx emissions. Free-piston engines, and more particularly free-piston engines of the single piston/single piston assembly type, are a natural fit for HCCI combustion: because such engines have inherently variable stroke, a compressing force may be applied indefinitely to a homogeneous charge of any temperature and equivalence ratio until the charge auto-ignites, and, because piston motion is unconstrained, the exact moment of combustion need not be controlled. This is in marked contrast to existing crank-piston HCCI engines, in which engine temperature, temperature of the intake charge, and equivalence ratio must be carefully monitored and controlled to ensure that spontaneous ignition occurs at or near top dead center of piston motion. Crank-piston HCCI engines additionally face the potential for rods and cranks to be damaged by the high pressure peaks characteristic of HCCI combustion: because of this, they are typically run at very lean mixtures and constant low loads. Free piston engines, by contrast, have no conventional rods or cranks, and can thus theoretically be operated across a wide power range at mixtures up to and including the stoichiometric ratio without damage to engine components.
While several recent designs utilize HCCI combustion in a two-stroke free-piston engine of the single piston/single piston assembly type (e.g., U.S. Pat. No. 6,199,519 and U.S. Pat. No. 6,959,672), none employs the engine as a combustor (gas generator) where the energetic output gas is used to drive a rotary motor. The first example above is configured as an electrical linear alternator, for instance, while the second assumes a configuration as either an electrical linear alternator or a hydraulic or pneumatic pump. All other known configurations of two-stroke, HCCI free-piston engines of the prior art incorporate one of these three power extraction methods. These configurations fail to take full advantage of the free-piston model for HCCI operation: because they attempt to extract useful work from the piston motion, piston speed and momentum are reduced, and the engine's ability to generate a compressing force sufficient for auto-ignition of the charge is thereby rendered problematic. In practice, most such engines require complex sensing and control mechanisms to balance compression force with power extraction forces, and they have thus been confined to constant-load operation. Also, because extracting work from piston motion reduces the maximum attainable operating frequency of the engine, such engines have exhibited poor power density. A superior solution is suggested by utilizing a power turbine or rotary gas-driven motor for power extraction in two-stroke free-piston HCCI engines of this type, but there is no known instance of such a design in the prior art.
The prior art does contain at least one instance of a two-stroke, HCCI, free-piston combustor (gas generator) in isolation, namely that described by J. Horton (“Amazing New Lightweight Turbine,” Mechanix Illustrated [February, 1969]: 66-68; 134-136). Because it is configured as a pure jet with no attempt made to extract work from the piston motion, the Horton engine enjoys several advantages over a similar engine configured as a linear alternator or hydraulic or pneumatic pump, including simplicity (no sensing or control mechanisms are required, and the engine has three moving parts), extremely high operating frequency, and very light weight. However, since the Horton engine is a pure jet, it has a limited range of applications (for instance, it is unsuitable for a land vehicle operated in traffic). Additionally, the Horton engine is reported to suffer from extreme noise, as well as from typical two-stroke disadvantages of needing to add lubricant to the fuel and of short-circuiting raw fuel out the exhaust. Finally, the Horton engine employs an engine geometry that requires very precise machining and that is nonetheless vulnerable to leakage and binding. All of these challenges are addressed by the currently preferred embodiment of the present invention.
Special mention should be made of two additional patents: U.S. Pat. No. 1,785,643 (referenced earlier) and U.S. Pat. No. 7,258,086. U.S. Pat. No. 1,785,643 (Noack et al.) describes a single-piston assembly, two-stroke, compound free piston-gas turbine engine in which an integral, reciprocating linear electric motor/generator is coupled to a rotary generator in order to obtain favorably-phased oscillation rates of the piston assemblies in multiple free-piston combustors (gas generators). While the Noack engine did not utilize HCCI combustion, the use of a linear motor/generator to obtain these favorably-phased oscillation rates is similar to that put forward in an alternative embodiment of the present invention, which will be described later in greater detail. U.S. Pat. No. 7,258,086 (Fitzgerald) describes a four-cylinder, four-stroke, HCCI, compound free piston-gas turbine engine as one alternative embodiment. While this stated embodiment shares several operating principles with the currently preferred embodiment of the present invention, numerous distinctions arise from the unique architecture required to support four-stroke vs. two-stroke operation. Relative to the compound free piston-gas turbine embodiment claimed in U.S. Pat. No. 7,258,086, the currently preferred embodiment of the present invention: 1) uses half the number of pistons and cylinders to obtain the same number of power strokes; 2) eliminates a moving piston linkage; 3) utilizes simple ports rather than a positive valving system; 4) employs an under-piston chamber to provide scavenging pressure rather than employing a separate intake stroke; 5) employs closed-cylinder fuel injection in preference to open; 6) employs a novel, closed-circuit lubrication system, and 7) provides for the optional substitution of a positive-displacement gas-driven motor and/or centrifugal turbine in place of the “power turbine” named in the Fitzgerald embodiment. A four-stroke architecture is one valid means of addressing common two-stroke problems of a narrow power band, poor fuel efficiency, and high emissions; however, the currently preferred embodiment of the present invention utilizes alternative means to address these problems and eschews a four-stroke architecture in favor of the higher operating frequency, improved power density, decreased complexity and cost, and reduced friction inherent in its two-stroke configuration.